The present invention relates generally to optical wavelength conversion and switching.
All-optical signal processing, such as wavelength conversion and switching, offers the potential for operation at much higher speed than what is currently possible with electronic devices. One application for all-optical devices is fiber communication networks where ever increasing data traffic is continuously fueling the demand for system elements that facilitate fast, efficient and reliable handling of information. One known approach for providing data to a selected destination includes wavelength division multiplexing (WDM) which allows for the simultaneous optical transfer of data at different wavelengths, thereby utilizing the optical bandwidth of contemporary silica fiber. Communication networks based on WDM require optical devices with which the data flow can be controlled and directed.
WDM based systems include wavelength converters which can be used for buffering and routing of data packets. Known types of wavelength converters include cross gain modulators (XGM), four wave mixers (FWM), cross absorption modulators (XAM), and cross phase modulators (XPM). Each of these types of devices has certain disadvantages. For example, for some of these devices, such as XGM converters, the output signal is inverted and exhibits a large chirp. Further, converters which are implemented in semiconductor optical amplifiers (SOA) require strong electrical biases. Also, some of these devices have disappointing signal to noise ratios due to signal amplification. Other devices may invert the signal and may not be cascadable.
It would, therefore, be desirable to provide an optical communication system utilizing wavelength division multiplexing that overcomes the aforesaid and other disadvantages.
The present invention provides a wavelength converter based on electromagnetically induced transparency (EIT). EIT refers to the elimination of resonant absorption on an otherwise optically allowed transition by the application of a coherent coupling light field in the presence of a probe field. Although the invention is primarily shown and described as an EIT wavelength converter for an optical communication system, it is understood that the invention is applicable to other systems in which transmission of light of one color is to be controlled with light of a second color. Furthermore, the wavelength converter of the present invention is not limited to the optical wavelength range of contemporary fiber communication systems.
In one aspect of the invention, an optical communication system includes an EIT based wavelength converter that forms a portion of an optical packet switch (OPS). In general, optical packet switches are used are used to control data traffic on the level of metropolitan and local area networks (MANs, LANs). The optical packet switches allow routing of data traffic between or within different MANs and LANs. The switches require buffering and synchronization of optical packets, which can be provided by EIT wavelength converters in accordance with the present invention.
In one embodiment, the wavelength converter includes an EIT medium having as least three energy states, e.g., |1 greater than , |2 greater than , and |3 greater than . A continuous wave (cw) probe field is applied on a first state transition, e.g., |1 greater than -|3 greater than , for which the EIT medium is optically thick. In the presence of a coherent coupling field that fulfills the conditions for EIT resonance, the EIT medium becomes transparent to the probe and coupling fields. Thus, the state of the coupling field, e.g., on or off, determines the output state of the probe field. A bit pattern on the coupling field, e.g., the input, can be converted to a wavelength corresponding to the wavelength of the probe field.
In a further aspect of the invention, an optical communication system includes an EIT wavelength converter that provides Nxc3x97N conversion. More particularly, a single EIT device can convert N data input signals, each having a respective wavelength, to N data output signals, each having a respective converted output wavelength. Nxc3x97N conversion is realized by applying pairs of probe and coupling fields to the EIT medium, each of which can provide transparency of the EIT medium. Bit patterns on the respective coupling fields can be converted to the wavelength of the corresponding probe field. It is understood that the phase differences between probe fields are substantially identical to the phase differences between the coupling fields.